This invention relates to the detection of heavy metal contaminants in a furnace in which semiconductor devices are to be processed. During their manufacture, semiconductor devices are repeatedly heated to elevated temperatures. The presence of heavy metal contaminants, for example, nickel, iron, gold or copper, in any one of the heating furnaces introduces heavy metal atoms into the semiconductor device being processed and these atoms cause an ofttimes undesirable decrease in minority carrier lifetime and an increase in leakage current of a reverse biased junction.
As part of the quality control function in semiconductor manufacturing operations, the heavy metal contamination in a furance is monitored so that corrective action can be taken if the furnace is contaminated. One such monitoring technique is to measure the reverse recovery time of the devices being processed after the wafer is removed from a furnace. If the reverse recovery time is lower than expected for the type of device being processed, it would indicate the presence of heavy metal contaminants. With this technique, however, monitoring can not begin until the PN junctions are formed. Thus, this technique is after the fact and needless expense may have been incurred in processing numerous wafers. Finally, since the devices being processed vary significantly, a data base of expected reverse recovery time for each device being processed must be maintained. Accordingly, the above technique is not entirely satisfactory.
Another technique for monitoring contamination is a surface photovoltage technique in which light is directed on the surface of the wafer to determine how much light is absorbed. The more light absorbed, the more heavy metal contaminants are present. This technique is not sufficiently sensitive to detect low contamination levels in the device. Another technique has been to measure the capacitance versus voltage of the device. This technique works for the detection of sodium contamination, but has not shown any reliable correlation with heavy metal contamination.